The problem addressed by the invention is to compare two optical surfaces, spheres or aspheres, which have nominally the same shape. This may done with a Fizeau-interferometer system by placing the first surface—called the master calibration surface—in front of the Fizeau reference surface and making a measurement of the resulting phase-difference of the light reflected off the Fizeau-reference surface and the master calibration surface. This result is stored. Next, the master calibration surface is replaced by the second surface called the test—surface. Then another interferometric measurement is performed on the phase-difference of the light reflected off the Fizeau-reference surface and the test surface.
Subtracting the second measured phase-difference from the first measured phase-difference delivers the wanted deviation of the test surface from the master calibration surface. Typical interferometric systems, such as the Fizeau configuration, are designed for testing in the null condition, or aligned perfectly on axis. In certain data acquisition systems and in practical use (e.g., carrier-fringe analysis), misalignments are either required or common. These misalignments cause wavefront errors commonly called ray-mapping errors. Such wavefront errors are often large enough to degrade the performance of the interferometer. The correction of these ray-mapping errors extends the practical application of interferometers.
Consequently, it is a principal object of this invention to quantify ray mapping errors that are proportional to the amount of misalignment error in an interferometric system and provide a method to correct these errors.
It is yet another object of the present invention to provide a calibrated interferometer employing carrier fringe analysis while compensating for errors introduced by tilt.
It is yet another object of the invention to provide methods for calibrating an interferometer system in a very straight forward, deterministic, very cost effective, and elegant way.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter when the following detailed description is read in conjunction with the accompanying drawings: